Currently, it is well known to provide heat treatment to metal implements such as handtools, vehicle parts, factory machinery, or any other metal product for which strength, wear, cyclic life, impact, or other materials characteristics determine the performance of the implement in its intended or expected usage. Virtually all metal products are made from an alloy or mix of elemental materials, the alloy typically having a dominant metal such as iron or aluminum that also is the dominant factor in the resulting performance characteristics of the alloy and any product made therefrom.
As examples, it is common to fashion a variety of handtools for driving or removing workpieces such as screws, nuts, bolts, and the like, these handtools commonly known as wrenches and socket driver sets, referred to herein collectively simply as drivers. Within the range of both intended usage and unintended though expected usage for such handtools are a need for a user to apply high torques through an elongated member including torque overloads, a need for a close fit with a workpiece for transmitting torque thereto, an expectation of high cyclic life before failure both in terms of wear and in terms of fracture, and an expectation of high impact life both in terms of cyclic impact life and single-event impacts. To be clearer, a user assumes that a tool will perform as intended with repeated application of torque, that contact surfaces have a high resistance to wear, and that the user will be able to mistreat the tool at times, such as by dropping or overtorquing the tool, or even using it as a hammer, as mere examples.
The important factors for providing a driver handtool as described are the material composition, the steps taken to form the tool, the actual shape of the tool considering stress concentrators, for instance, and the heat treatment. The most common material for drivers is a steel alloy of iron, carbon, and relatively small amounts of other metals. Formation of the tool in part or in whole may be by any method or methods, such as forging, casting, stamping, or rolling, etc., and generally includes some stage of cold-working in order to shape a working end for engaging a workpiece. More specifically and as an example, it is generally practiced to cut a socket recess into a socket driver for receiving a bolt head or extension socket post.
The overwhelming dominant industry-practice for manufacturing such drivers is to provide stock, cold-work the stock to a desired driver shape, and then heat-treat the driver. A first process for heat treatment is hardening including heating the drivers into the austenitic range and then quenching (either water spray or an oil bath) which results in a martensitic structure for the steel. Additional carbon may be added during this heating step, which is known as case-hardening. A second process in the heat treatment is a tempering stage including again heating the driver or tool to a temperature below the austenitic transformation temperature, holding the temperature at such level for approximately 2 hours, and then allowing the tool to cool slowly, such as with ambient air. Accordingly, the entire tool has a tempered martensite structure.
Like many metal implements, it is known that there are performance trade-offs when providing a heat treatment for such drivers. For instance, it is known that a high hardness for a steel product imparts high resistance to wear, yet also results in susceptibility to damage from impact or sudden load spikes, as such steel is brittle. In order to relieve or reduce the fragility of the tool, greater ductility may be imparted that results in the tool having a lower yield strength and, thus, being softer, more susceptible to wear, and more susceptible to plastic deformation. The heat treatment and materials performance characteristics resulting from the heat treatment for tools such as drivers are selected to balance these characteristics and tendencies.
Accordingly, there has been a need for an improved heat treatment for tools, particularly drivers and other elongated handtools.